Exactly which N items need to be collected is determined by the item table at 41984. When N=1, the only item that needs to be collected is the one in The Bathroom.
Note that the routine at 35211 tries to draw all remaining lives at the bottom of the screen, which means that for large N, there is screen corruption and the game slows down considerably; to avoid this, disable the routine:
When it became clear that the Attic bug was actually a bug and not a feature, Software Projects published some POKEs to fix it and some other problems with the game:
The second POKE (42183,11) moves the unreachable invisible item at (3,26) in First Landing to the same coordinates in The Hall (where it is still invisible, but now reachable).
The third POKE (59901,82) fixes the misplaced arrow in The Attic by setting its pixel y-coordinate to 41.